How to disable Access Log for a single vhost

[Lnxgs]

Hi,
I would like to disable access log only for a unique virtualhost (the others must have it).
I put this:
CustomLog /dev/null common
on additional HTTP directive on Plesk GUI for such site and I restarted apache web service.
However, I continue to see that "/usr/sbin/apache2 -k start" makes use of the virtualhost access_log and logs are regularly written on this file.

How to disable this log effectivelly?

The O.S. is Ubuntu 14.04.2 LTS and the Plesk version is 12.0.18.

Regards,
L.